1. Field of the Invention
The present invention relates generally to automatic inspection equipment, and more particularly to a portable laser-based inspection system for automatically scanning the interior surface of a refractory lined vessel in order to determine the wear characteristics of the vessel lining. The invention may also be used to inspect the interior of vessels which have been allowed to fully or partially cool as well as the interior of other types of vessels such as electric furnaces, ladles and the like. The vessel can be of the type used to contain ferrous, nonferrous or other molten materials.
In one process for making steel, molten iron, with a high carbon content, is refined in a basic oxygen furnace (BOF) by blowing oxygen into the molten iron from the top of the furnace vessel by means of a water-cooled lance. This is a violent exothermic process which results in erosion of the lining of the furnace vessel.
The lining in this type of BOF vessel generally consists of an approximately three foot thick layer of ceramic brick inside a steel shell. This refractory material protects the steel wall of the furnace shaped vessel from the about 3,000.degree. F. (1650.degree. molten steel during the refining process. The refractory brick lining eventually wears away due to the constant bombardment from scrap charging and the corrosive nature of the molten steel-slag combination.
To prevent removing the vessel from operation to repair worn brick, it is common practice to spray a refractory slurry on the damaged areas of the vessel between heats. In using this repair process, it is important to know the topography of the vessel lining to guide the repair process. This may be accomplished by measuring the surface topography of the interior wall of the vessel at a particular point in time, measuring the topography of the interior wall surface at a subsequent time, and comparing the differences to determine the change in thickness of the vessel refractory lining.
In many instances, the carbon content of the steel and the temperature required for a particular steel chemistry are achieved by top-blowing oxygen through a lance, the lower end of which is positioned a predetermined distance above the upper surface of the liquid steel bath. If the lance is positioned too low, lance life is adversely affected. Conversely, if the lance is positioned too high, steel quality and yield are adversely affected.
In addition, many BOF vessels utilize bottom stirring wherein inert gasses are blown through tuyeres installed in the refractory lining. The use of such bottom stirring methods has generally made the conventional methods of measuring steel bath height within the vessel unreliable. For example, one typical method for measuring bath height involves inserting a metal rod into the molten bath and measuring the remaining length after the portion in the bath is melted. However, where bottom stirring is utilized, the turbulence created by the bottom stirring gasses generally makes this measurement technique unreliable.
As is well known, steel bath level depends not only on the quantity of material charged into the vessel, but also on the topography of the refractory lining. The present invention also includes means for accurately and quickly determining the level of the molten steel bath in the absence of stirring gasses. In the preferred embodiment to be described in more detail hereinafter, this is accomplished by determining the coordinates of a number of points on the refractory surface inside the vessel, and calculating from these points the interior volume of the vessel as a function of steel bath height. The unstirred or flat bath level can then be determined as a function of steel charge weight.
2. Description of Related Art
One type of measurement device for determining the wear characteristics of the interior lining of the furnace vessel is described in U.S. patent application Ser. No. 560,915 entitled "Apparatus for Measuring Wear in the Lining of Refractory Furnaces", filed Dec. 13, 1983, and assigned to the assignee of the present application. That system uses a laser triangulation system where a laser beam is directed at the hot refractory wall. Scattered laser radiation is then collected by a lens and focused on a linear array of photodiodes in a self-scanned camera. The address of the diode where the image of the laser spot is focused is proportional to the distance to the refractory wall. As the wall distance varies, so does the location of the laser image on the array. A narrow bandpass interference filter is used to detect only the laser wavelength, while ignoring the background radiation emitted by the hot wall. The distance to the vessel wall can then be correlated via a computer with the actual remaining lining thickness.
This type of scanning system is lowered into the hot furnace vessel at the end of a water-cooled lance. The vessel is in a vertical orientation for this inspection. The sensor then mechanically scans the inside of the vessel for damaged areas. The lance rotates through 360.degree. and indexes up or down to enable a complete mapping of the vertical walls of the vessel. This type of system may be controlled by an operator's console in the pulpit.
For environmental reasons, however, many furnace vessels are provided with an overlying collection hood or fume scrubber which collects fumes and gases issuing from the open top of the furnace vessel. Consequently, a vertical entry laser inspection system is impractical for these types of installations.
One prior art system designed to inspect this hooded type of furnace vessel is described in U.S. Pat. No. 4,508,448 entitled "Apparatus For Measuring The Distance To A Point On The Inner Wall Of A Hot Furnace". This system uses a laser time of flight phase measurement technique to determine the wall position. The system is totally manual in that it requires measurement of building and vessel reference points, which requires a substantial set-up time. In addition, once the unit is positioned, the operator must manually move the head to direct the laser beam to each point on the lining to be measured and then focus the head on that point. This requires a substantial amount of time to take each reading. Typically, 45 minutes to 1 hour is required to completely inspect the interior of a vessel using this type of system. In addition, this type of system may lack sufficient environmental controls so that the unit cannot be positioned sufficiently close to the open top of the still hot horizontally disposed furnace vessel. While this can be overcome somewhat by placing the unit further back from the furnace, the measurable field of view within the vessel is significantly reduced. Further, this type of system only produces a series of points along one vertical section or "slice" of the interior of the furnace vessel. Consequently, there is lacking much information about the interior contour of the vessel.